Design of Welded Structures by Blodgett

5.1-12

/

Welded-Connection

Design

Weldor makes continuous beam-to-column connection on Inland Steel Co.'s office building in Chicago. At this level, the column cross-section is reduced, the upper column being stepped back. Spandrel beam is here joined to column by groove welds. The weldor, using low-hydrogen electrodes, welds into a backing bar. Run-off tabs are used to assure full throat size from side to side of flange.

For New York's 21-story 1180 Avenue of the Americas Building, welded construction offered important weight reductions and economy, quiet and fast erection. Maximum use of shop welding on connections minimized erection time.

5.5-6

/

Welded-Connection

Design

fw = 11,200 CI)

Force on top plate is-

= 11,200 (%6) = 3500 lbs per linear inch

F -~ -db in.-kips) = (900 (14.12")

The length of this weld isF

= 63.8 kips

Lw=-

fw -(.656

in.2) (36,000 psi)

-(3500

lbsjin.)

The top plate is designed for this force at Ih higher allowables:

F

=~

Ap=~

This would be 10/4"across the end, and 2%" along the sides. Applying Method 1 lor Additional

Wind Moment

This connection will now be designed for the additional wind moment of Mw = 600 in.-kips, using Method 1. I. 5" .1,,2Y2': I

=

(63.8 kips) . IIh (22,000 pSI)..

= 2.18 in.2 or use a 3'L" T~ x~M" plate Ap = 2.19 in.2 > 2.18 in.2

Q!

The connecting welds are figured at Ih higher allow-

abIes: b

For the fillet welds at the beam flange, use %" fillets. The standard allowable force is fw = 11,200 CI) = 11,200 (%) = 5600 lbs per linear inch. The length of this weld is-

F

jF

(63.8 kips) = IIh (5600)

...,L-

= 8.54"

FIGURE10 B

di . h earn con lions ere: 14" WF 38# beam b = 6.776" db = 14.12" tt = .513" ..800",.. S = 54.6 m.3

Total moment on the connection isM = Mg +

Mw

= 300 in.-kips + 600 in.-kips = 900 in.-kips

Lw=~

db = 14.12" 1 F

This weld length would be distributed 3%" across the end, and 2~" along the side edges of the top plate. The above connection may be cut from bar stock without the necessity of flame cutting any reduced section in it. This is a good connection and is in widespread use. The connecting groove weld and fillet welds are strong enough to develop the plate to yield, plastically if necessarydue t 0 any accid enta1 overI oad 0f the

.

connection. Some engineers prefer to widen this plate at the groove weld so that if the plate should have to reach yield stress, the connecting welds would be stressed only up to the wind allowable or Ih higher, hence 0" = Accordingly, the plate is widened here to 1¥4W= 1¥4(3~) = 4%". (See Figure 11.) The length of the fillet weld, using %" fillet welds and allowable of fw = 5600 lbsjin., would be-

5.8-4

/

Welded-Connection

Design

FIGURE 8

stress.This may be done by one of severalmethods, Figure 8. (2) Now assumethe girder to be fixed at the ends and the beamswelded for continuity to the girders.

-.90 M 0"1-S .90 (1500 in.-kips) = ( 62.7 in.S) = 21,500psi (Only need S = 56.2 in.s, but this is the lightest 14" WF section.) M2 = + M1 48 L

- + (6Ok)(240") -48

FIGURE 9 Design the girder as having fixed ends. Use 14" WF 43# beamhavingS = 62.7in.s

~

~,~20"20"

20k

= 4780 psi WL Ms = + 16

20' 111111~~:IIIIn@DJ~"'11111 IJJJY~ 0 ~

G) 1

= + 300 in.-kips M2 0"2=S (300 in.-kips) = (62.7 in.S)

Moment diagrom

-+ (OOk)(24O") -16 = + 900 in.-kips Ms

O"s= S M -5 1 --48

W1 L

= -1500 in.-kips

-5(6Ok) --48'

(240")

-(900 in.-kips) -(62.7 in.S) = 14,350psi

7.1-4

/

Joint Design and Production

A441 specifies the same strength requirements A242. The chemical requirements limit carbon manganese to the same levels per cent minimum vanadium

as and

S. HIGH-STRENGTH Proprietary Grades

as A242, but add 0.02 to obtain the desired

Proprietary

grades

LOW

of high-strength

strength levels without the need for more expensive alloy additions. As in the case of A440, the Specification limits the sulphur and phosphorus, and requires that the steel be "copper bearing" to improve its corrosion

available which are grades but differ in specified minimum psi to 65,000 psi.

resistance

used in manufacturing,

over that- of A7.

TABLE

1 B-A

Comparison

of

Steels

AST M High-Strength

ASTM Grade

Min. Yield Point psi

Thickness Shapes

Group

I

(1)

Group II (1) A440

50,000

70,000

46,000

67,000 mln;

Group III (1)

42,000

63,000 mln;

50,000

70,000min.

Ba~

Over %" to l1f2" incl.

46,000

67,000 min.

Over 11/2" to 4" Incl.

42,000

63,000 min...

(1)

50,000

70,000 min.

Group II (1)

46,000

67,000 min.

&

Bars

low alloy steels are

similar to the ASTM high-strength certain respects. These steels have yield points ranging from 45,000 Although these steels are widely they have

only

recently

Construction

Steels

C Max.

p Max.

Mil

S Max.

Si Max.

Cu Min.

Group III (1)

42,000

63,000 min.

To 0/4" Incl.

50,000

70,000 min.

Over 3/4" to l1f2" Incl.

46,000

67,000 min.

Over 11/2" to 4" Incl.

42,000

63,000 min.

Over

40,000

60,000

(1)

50,000

70,000 min.

,

46,000

67,000 min.

42,000

63,000 min.

4"

to 8"

Shapes Group I

incl.

Group II (1)' Group III (1)

A242

'!

c'

.28

To

& Bars

%"

Incl.

~

1.10/1.60

.04(2)

.05

1! ,, .20

.30

70,000

Over 0/4~~to 1.'/2" incl.

46,000

67,000 mIll.

Over 11/2" to 4" incl.

42,000

63,000 min.

,

'...

.30

.20

,

.22

1.25 max.

.04

.05

,;,ii

(

(3) ;

:\

,

"l

J:~' .",:

f 4'

Group II

Group III '"

Wide Flange Shapes

Wt. per ft., Ib:

Mominal Sie*, in;

All weights All weights

14 x 16

12 x 12

)c

i'

142 to 211 Inel. f

120 to 190 Ilici.

Angles aver %" thick *Naminal depth and nominal width of flange (2) Basedan basic steelmakingprocess. (3) Thechoiceand useof allaying elementsto producethe required strengthor to improvecorrosionresistance, or bath, will vary with the manufacturer.

I

Wt. per ft., lb.

I

210 to 4~& Incl.

'. ;

14 x 16

i..

,. ,c [\

"

except those..

listed In Groups II & III

,

-

max..05

min.

"

All shapes

.02

,

, 1.25

Wide Flange Shapes

36 x 161f2 33 x 150/4 '

1

,. "

(1) Groups I, II, III are defined as fallows:

Nominal Si%e*,ih.

'!

min.

50,000

Group I

Other

-"

.22 Plates

V Min.

r

,

c

begun

min.

To %" Incl.

Plates

STEELS

Chemical Requirements (Ladle) Per Cent Tensile Strength psi

Plates

Shapes Group I

A441

-"-

for

ALLOY

I

"

~

7.1-6

/

Joint Design and Production

to 100,000 psi, and ultimate strengths ranging from 105,000 to 135,000 psi, depending upon thickness. Originally these steels were available only in plates

of these steels in construction occur when unusually high loads are encounterep, particularly in tension members. i

because of difficulties encountered during heat treating in maintaining the straightness of shapes. By 1961 many of these difficulties had been overcome, and these steels are now offered in certain structural shapes. Because of the higher price of these steels, their use in building construction has so far been rather limited. However, they have been used to considerable advantage in several large bridges built in recent years, and in other types of structures. The major applications

Heat-treated constructional alloy steels have the ASTM designation of A514-64. Where local codes permit the use of these steels and when loads are of sufficient magnitude, and tension loads are encountered or lateral buckling is restrained, economies can be gained through the use of the heat-treated constructional alloy steels.

B. SELECTING THE RIGHT STRUCTURAL STEEL 7. BASIS FOR SELECTION

A36 is the best buy for construction purposes.

With the adoption by the AISC of design specifications covering the use of six ASTM steels (A7, A373, A36, A440, A441, and A242), designers are now able to choose the particular steel which is best suited to the job at hand. However, before designers can take advantage of these steels, some insight must be acquired as to where each can be used to the greatest advantage. To aid the designer in this selection, we shall compare the five ASTM steelsrecommended for welded construction on the basis of price, and also on what we call "yield strength per dollar". We shall also present guides to aid in recognizing those situations wherein the use of high-strength steels has proven to be advantageous.

High-Strength

8. COMPARISON BASED ON PRICE " Price is, of course, a factor in the selection of a steel. Table 2A (for shapes) and Table 2B (for ,l:)lates) show the comparative prices of the five ASTM structural steels and proprietary high strength, low alloy steels. Car bon SteeIs In carbon steel shapes, A36 steel is the same price as A7, has a 10 per cent higher specified minimum yield point, and can be welded with high speed, low cost procedures. The maximum carbon content is only 0.26 per cent. A373 has a higher maximum carbon content (0.28 per cent), a higher price, and a lower yield strength than A36. In shapes, therefore, A36 is by far the best bargain of the carbon steels. In plates, the advantage of A36 is not quite as pronounced as in shapes. However, because of its higher specified minimum yield point, relative ease of welding, and the requirement that the steel be produced fully killed in thicknesses over 1% inches thick,

Steels

In the high strength steels, for material thicknesses up to 0/8"inclusive, A441 is the same price as A440. For thickness over O/S" to 3!4"inclusive, A441 is only slightly more expensive than A440. Since A440 steel is not generally recommended for economical welding, A441 is a more versatile and useful steel for construction purposes. The A242 grades are substantially higher in cost than A441. Consequently, it would be uneconomical to use A242 unless improved corrosion resistance is desired. If this property is desired, it should be so specified; mere reference to the A242 specification does not assure improved corrosion resistance. 9. COMPARISON BASED ON YIELD STRENGTH PER DOLLAR Price alone does not always give an accurate picture of the possible cost advantage of one steel over another, particularly where a difference in yield point is involved. Table 3A (for shapes) and Table 3B (for plates) compare the five ASTM structural steels on the basis of comparative yield point per dollar of cost, with A36 steel used as the basis for comparison. Although such a comparison gives a more accurate picture than a comparison of price alone, a comparison of steels on the basis of the strength-to-price ratio must be made with the following qualifications: a. Strength-price values are based on minimum yield point. Where factors other than yield point (such as limitations due to deflection, buckling or lateral stability) determine the allowable stress,strength-price values based on minimum yield point are not a valid comparison.

7.2-4

/

Joint

Design and

Production

Plate is later preheated,

Hardened zonein ')

Tackweld without

base plate',...

preheat

~

and submerged-arc weld will remelttack weld and hardenedzone in plate

(a)

(b)

FIGURE 1

Factors that Affect

Weld Cracking

During

Welding

1. Joint Restraint that causes high stresses in the weld. 2. Bead Shape of the deposited weld. As the hot weld cools, it tends to shrink. A convex bead has sufficient material in the throat to satisfy the demands of the biaxial pull. However, a concave bead may result in high tensile stresses across the weld surface from toe to toe. These stresses frequently are high enough to rupture the surface of the weld causing a longitudinal crack. An excessively penetrated weld with its depth greater than its width under conditions of high restraint may cause internal cracks. Both of these types of cracking are greatly aggravated by high sulphur or phosphorus content in the base plate. 3. Carbon and Alloy Content of t~ base metal. The higher the carbon and alloy content of the base metal, the greater the possible reduction in ductility of the weld metal through admixture. Thts contributes appreciably to weld cracking. 4. Hydrogen Pickup in the weld deposit from the electrode coating, moisture in the joint, and contaminants on the surface of the base metal. 5. Rapid Cooling Rate which increases the effect of items 3 and 4. Factors that Affect Zone

Cracking

in the Heat-Affected

Factors that Affect

Welded Joints Failing in Service

Welds do not usually "crack" in service but may "break" because the weld was of insufficient size to fulfill service requirements. Two other factors would be: 1. Notch toughness,* which would affect the breaking of welds or plate when subjected to high impact loading at extremely low temperatures. 2. Fatigue cracking* due to a notch effect from poor joint geometry. This occurs under service conditions of unusually severe stress reversals. Items

to Control

1. Bead Shape. Deposit beads having proper bead surface ( i.e. slightly convex) and also having the proper width-to-depth ratio. This is most critical in the case of single pass welds or the root pass of a multiple pass weld. 2. Joint Restraint. Design weldments and structure to keep restraint problems to a minimum. 3. Carbon and Alloy Content. Select the correct grade and quality of steel for a given application, through familiarity with the mill analysis and the cost of welding. This will ensure balancing weld cost and steel price using that steel which will develop the lowest possible overall cost. Further, this approach will usually avoid use of inferior welding quality steels that have excessively high percentages of those elements that always adversely affect weld quality-sulphur and phosphorus.

1. High carbon or alloy content which increases hardenability and loss of ductility in the heat-affected zone. (Underbead cracking does not occur in nonhardenable steel.) 2. Hydrogen embrittlement of the fusion zone

Avoid excessive admixture. This can be accomplished through procedure changes which reduce penetration (different electrodes, lower currents, changing

through eld

.Neither notch toughness nor fatigue cracking here. See Section 2.1, "Properties of Materials,"

w

m

migration etal

of

hydrogen

liberated

from

the

."Designing

3. Rate of cooling

are discussed Section 2.8,

for Impact Loads, and Section 2.9, "Designing

which

controls

items

1 and 2.

Fatigue Loads."

for

7.2-6

/

Joint

Design and

Production

a thinner plate, and since the thicker plate will probably have a slightly higher carbon or alloy content, welds on thick plate (because of admixture and fast cooling) will have higher strengths but lower ductility than those made on thinner plate. Speci~ welding procedures may be required for joining thick plate ( . .Molten espeClall y f or the fir st or root pass,) and pre heatIng' may be necessary.The object is to decreasethe weld's rate of cooling so as to increase its ductility. In addition to improving ductility, preheating thick plates tends to lower the shrinkage stressesthat develop because of excessiverestraint. Because of its expense, preheating should be selectively specified, however. For example, fillet welds joining a thin web to a thick flange plate may not require as much preheat as doe$ a butt weld joining two highly restrained thick plate$. On thick plates with large welds, if there is metalto-metal contact prior to welding, there i$ no possibility of plate movement. As the welds cool and contract, all the shrinkage stress must be taken up in the weld, Figure 2( a). In casesof severerestraint, this may cause the weld to crack, especially in the first pass on either side of the plate. By allowing a small gap between the plates, the plates can "move in" slightly as the weld shrinks. This reduces the transverse stresses in the weld. See Figures 2(b) and 2(c). Heavy plates should always have a minimum of %2" gap between them, if possible Yi6". This small gap can be obtained by means of: 1. Insertion of spacers, made of soft steel wire between the plates. The soft wire will flatten out a$ the weld shrinks. If copper wire is used, care should be taken that it does not mix with the 'feld metal. 2. A deliberately rough flame-cut edge. The small peaks of the cut edge keep the plates arart, yet can squash out as the weld $hrink$.

b) weld

FIGURE3 3. Upset;ting the edge of the plate with a heavy center punch. This acts similar to the rough flame-cut edge. The plates will usually be tight together after the weld has cooled. . FIllet Welds The above discussion of metal-to-metal contact and shrinkage stresses especially applies to fillet welds. A slight gap between plates will help assure crack-free fillet welds. Bead shape is another important factor that affects fillet weld cracking. Freezing of the molten weld, Figure 3(a), due to the quenching effect of the plates commences along the sides of the joint (b) where the cold mass of the heavy plate instantly draws the heat out of the molten weld metal and progressesuniformly inward (c) until the weld is completely solid (d) . Notice that the last material to freeze lies in a plane along the centerline of the weld. To all external appearances, the concave weld ( a) in Figure 4 would seem to be larger than the convex weld (b). However,' a check of the cross-

FIGURE4

a

oncove

I et we

onvex

we

7.2-8

Joint

Design and Production



-C.fttol

{~~t==~~?~~~~~Jr "

--'-I"

t ea t

oto.!."

\

~ax. W81d Both '101 t,t

B-Pla x.

"

g:::~~'-

T '

t

t -1".'

I"

t=

0

-.j f.-f~min.

te=td

B-P2

C-P2

t >"7.1.-

"

t=2M

.-,,;"'. td

0

t 0 1I

in.

.1. . 32,",n.

t

B-P4

45° min.

1

01

te:td-j

t. = t

B-P04 It> i II

o~\...

II

4Somin.

0

~,-

t> ~

te= td

IT

B-P8

TC-P8 ~o . 4.. "'",.

AfJomi".

\

7

1d ',~'

r -I

'T

-4

0

t.= td 8-P6

NOTE:

C-P6

1. Gouge root before welding second side (Par 505i) 2. Use of this weld preferably limited to base metal thickness of 5/8" or larger. .When lower plate is bevellet1, first weld root pass this side.

3. TYPES OF JOINTS

bevel, J, or U. Certain of these joints have been pre-

The type of joint to be made depends on the design condition and may be one of the following: groove, fillet, plug or T joint. These joints may be made using various edge preparations, such as: square butt, Vee,

qualified by the American Welding Society ( AWS ) and are illustrated in two charts, Figure 16 for manual welding and in Figure 17 for submerged-arc automatic welding. The choice between two or more types of joint

,

7.3-10

/

Joint

Design

and Production

~

.-.I.~ f § a;~.i~;

~,~ !~ ~ ~.i~i:~! i!!. ~'IE~~!! ~.~i i;"§ i; ! ~;I~!i ie ; T

~

Cl) .J

0 In

~

~

~

>

~.~

Cl)

5~ ;

0

:~i ..~i

Z -gii;~

ii..

0 d

w ~

::>

A~ t ~ .w

FIGURE 10

1. If t IE ~ %" \ FIGURE8

then tw = tWo ~

(AWS Bldg Art 212(a)3, AISC 1.17.6) Le ~ W

2. If t~ > %" then tw ~ ;2 t~ ~ %" Spacing and Size of Plug Welds (A WS Bldg Art 213, AWS Bridge Par 218, AISC , 1.17.11)

W ~ 8" unless additional welding prevents transverse bending within the connection. ~

-s ---"-

addition, the effective length (L.) of an intemlittent fillet

weld shall not be less than 1th" (AISC 1.17.7).

-"

~

"-

" "

--~~p;)~- --"-

--

--"-

"

FIGURE 11

7.4-8

/

Joint Design and Production

TABLE 6-Allowables for Welds-Buildings (AWS Bldg & AISC) Type

of Weld

Stress

CompletePenetration Groove Welds

tension compression shear

Steel A7, A36,

Electrode

A373

Allowoble

:t:E60 or SAW-l same as

A441,

A242*

A7, A36, tension transverse to axis of weld

A441,

'E70

A373

A242

*

t

or SAW-2

E60 or SAW-1 E60 low-hydrogen or SAW-1

U

or

T -13600 -,

' pSI

or shear on effective throat PartiolPenetration Groove Welds

A7, A373

E70 or SAW-2

A36

E70 or SAW-2 U or T =

A441

,

A242*

tension parallel to axis of weld or

A7, A36,

compression

A441

effective

on

A373

A242*

A441, shear

~~i~

A373

A242*

or

SAW-l

'

throat

Plug

shear

effective area

T =

13,600 or

E70 or SAW-2

A36

E70 or SAW-2

T =

15,800

E7~rl~'o:;:-;~rogen

f =

11'::00

A242*

psi

'" Ib/

f --In 9600

A7, A373

A441,

and Slot

E60 or SAW-1 E60 low-hydrogen or SAW-2

on

effective

* weldable

E70

do IL

throat

A7, A36,

,

psi

:t:E60 or SAW-l same as

or

15,800

E70 low-hydrogen or SAW-2

'

psi '"

Ib/ln

on

Same as for

fillet

weld

A242

:t: E70 or SAW-2

could

be used, but would

not increase

allowable

TABLE 7-Allowables for Welds-Bridges (AWS Bridge)

.

Type

of Weld

Stress ~

Steel

Electrode

Allowable

A7, A373 :t:E60 or SAW-l

Complete-

tension

A36

~-

1" thick

Penetration Groove Welds

compression shear

A36

>

1" thick

".

,.

.",

.'-'

c':""

,

Fillet Welds

shear on effective

A441

'

A242*

:t:E60 low-hydrogen or SAW-I

Same as

fE.

E70 low-hydrogen or SAW-2

A7, A373 A36

~ 1" thick -or

A36

>

1" thick

:t:E60 or SAW-l

T -,-

:t:E60 low-hydrogen or SAW-l

f =

8800 '" Ib/ln

T =

14,700 psi or 10,400 '" Ib/in

12 400

ps I

throat A441,

Plug

shear

and Slot

effective area

* weldable

on

~;6

A242*

A3;f3;"

could

be used,

but would

thick

-12,400 A36 > 1" thick A44J, A242*

A242

:t: E70 or SAW-2

E70 low-hydrogen or SAW-2

not increase

allowable

f =

:t:E60 or SAW-l psi :t:E60 low-hydrogen or SAW-1

7.4-14

/

Joint Design and Production

For this reason the size of intennittent fillet weld used in design calculations or for determination of length must not exceed 2/3of the web thickness, or here: 213of 1/2" (web)

= .333" .to T~e. perce?tage of contI.nuousweld length needed for thIS mtennittent weld will be% =

. contInuous .' mtermittent

1 .The eg SIze 1 .12" eg SIze

present %" 1\ is welded in lengths of 4" on ... centers, or 33% of the length of the )omt, reducmg .t\. .

the leg SIze down to %6" ~ or % of the preVIOUS weld. This would require the percentage of length of joint to be increased by the ratio 6 / 5 or 33% (%) = 40%.

-i~ -(.333") = 46% Hence use,

Hence, use-

1/2" t\ 4" -8",(see I I

D._"lftProblem

Table 10)

%6" t\ 4" -10"~

~ I 3 I

In other words, %" intennittent fillet welds, 4" long on 12" centers, may be replaced with %6" welds,

4" long on 10" centers, providing same strength. This change would pennit welding in one pass instead of two passes,with a saving of approx. 16213% in welding

A fillet weld is required using , %" f\ 4" -12"

I

that is, intennittent welds having leg size of %" and length of 4", set on 12" centers. A 0/8"fillet weld usually requires 2 passes, unless the work is positioned. A 2-pass weld requires more inspection to maintain size and weld quality. The shop would like to change this a %6" weld. This single-passweld is easier to make and there is little chance of it being undersize. This change could be made as follows:

"-

time and cost.

Problem 4 1

Determine the leg size of fillet weld for the base of a signal tower, Figure 22, assuming wind pressure of 5" stdpipe

\

t376.5"

20" dio Bose .-5" 40" dlO

30 Ibs/sq ft or pressure of p = .208 psi. Use A36 Steel & E70 welds.

~ 0

d1-6Vs d = 20.5"

20" dio

Ll

6" stdpipe

FIGURE 22

7.4-20

/

Joint Design and Production

11. WELDS SUBJECT TO COMBINED STRESS Although the (1963) AISC Specifications are silent concerning combined stresses on welds, the previous specifications (Sec 12 b) required that welds subject to shearing and externally applied tensile or compressive forces shall be so proportioned that the combined unit stress shall not exceed the unit stress allowed for shear. Very rarely does this have to be checked into. For simply supported girders, the maximum shear occurs near the ends and in a region of relatively low bending stress. For built-up tension or compression members, the axial tensile or compressive stresses may be relatively high, but theoretically there is no shear to be transferred. In the case of continuous girders, it might be well to check into the effect of combined stress on the connecting welds in the region of negative moment, becausethis region of high shear transfer also has high bending stresses. EvePAin this case, there is some question as to how mucj a superimposed axial stress actually reduces the shear-carrying capacity of the weld. Unfortunately there has been no testing of this. In general, it is felt that the use of the following combined stress analysis is conservative and any reduction in the shear-carrying capacity of the weld would not be as great as would be indicated by the following formulas. See Figure 28. In Figure 28: T = shear stress to be transferred along " throat of

From these formulas for the resulting maximum shear stress and maximum normal stress,the following is true: For a given applied normal stress (0"), the greatest applied shear stress on the throat of a partialpenetration groove weld or ffilet weld (and holding the maximum shear stress resulting from these combined stresseswithin the allowable of T = 13,600 psi for EOOwelds, or T = 15,800 psi for E70 welds) isfor E60 welds or SAW-1

I '1" ,--.,:=;./13,0002 ~{~:;;;;~ -~ .Lo.>,ovv-T

I

(7a)

for E70 welds or SAW-2

IT T ~~-;;;:~ :=; -r 15,8002-~ -., .Lu,OV\r-4

I

(7b)

This same formula may be expressed in terms of allowable unit force (lbsjIinear inch) for a fillet weld: for E60 welds or SAW-1

I ff ~~ ..~ Cl)./96Q02~-~, ~OVV--8

I

(8a)

for E70 welds or SAW-2

I ff:=;~ ~12 002 -8 .,~-;:;: Cl) 11,200

(8b)

weld, psi I 0"

=

norma

t s

Ii ress

app

d e

all par

I

t

e

0

~. aXIS

f 0

ld we

.the pSI From the Mohr's circle of stress in Figure 28:

~;~.~-:-~:-~ )

I

100max=~1I vmax T-=~~-2

Tmax=

' ~110"1\2 I _21 T 1~I \2)- 21-T32 -r '1"3-J

"

(2)T3~~ )21--r T32 2

For

the

same

given

applied

normal

stress

(0"

),

,

greatest applied shear stress (T) on the throat of a groove weld or fillet weld (and holding the maximum normal stress resulting from these combined stresses within the allowable of 0" = .00 O"y) is-

(6)

~ VtInA_'? (.00 #7 O"y)and-2 #8 -~.6v (InA~'0 expressed are O"y} ) 0" (9) 'I TFormulas InA_' _I I in table form, tInA_)?

(7)

as in Table 11. The general relationship of these formulas is illustrated by the graph, Figure 29.

_

7.5-4

/

~ " c ~u

Joint Design and Production

~ ~ 60.

of Weld Metal (Ibs/ft of Joint)

-.,;

:"':ct 'c

.

30.

'

"" "

".£""l;, !

~ ~

D

TABLE 3-Weight

I

"'

,

30.

- Ii ""

..-Ii'

,,-

+ 10%;.

t

,,;;~"

+ 0\

,

.+

os' " .,.

I

+10%

1

""",,011.. PO"

1 11/8 11/4 13/8 1 1/2 15/8 13/4 l 21/8 2 1/4 23/8 21/2 25/8 23/4 3

I + 10%

and_,no '.q""od "'.

300 reinforcement

5/8 3/4 7/8

1/3

I

0 4:

+

.0'

I

.456 .811 1.26 1.82 2.48 3.24 4.11 5.07 b.14 7.30 ';94. 11.4 1;s~0 14.7 1~.4"18.3 20.3 24.6

~o

300

reinforcement

reinforcement

200

.364 .544 .649 :; .735 1.01" 1.01 1.46 1.33 1.~ 1.62 2.60 1.93 3.28 !.26 4.06 2.62 4.91 3.01 5.~ 3.41 7.~ 4.29 9.12 4.75 10;4 5.25 11.7 5.77 13..1 8.31 14.7 6.88 16.21.46 19.6 8.71

reinforcement

.452 .626 ;830 1.06 1.30 1.56 1.83 2.13 ~:45 2.7~ 3.52 3.~1 4.32 4.7$ 5.20 5.67 6.16 7.20

200

300

200

reinforcement

reinforcement

reinforcement

2.53 3.02 3.54 4.07 4.63 5.i9 5.80 6.41 '.066.02 ;.72 '.11 9.85 10.6 11.4 12.2 c 13.0 13.8 i5.5

1.96 2.40 2.86 3.34 3.84 4.35 4.89 5.45

1.33 1.71 2.1. 2.bl 3.13 3.70 4.30 4.96 5.66 6.40 8.03 8.91 9.83 10.8 11.8, 12.9 14.0 16.3

6.62 7.85 8.51 9.18 9.87 .10.6 11.4 12.1 13.6

~~ ~~~~ TABLE 4-Weight

!

tC!

~ .~

-05.

-f

300

reinforcement

'O%W

'O%W

tC! -60.

of Weld Metal (Ibs/ft of Joint)

'=

+ 10\W

OS'

I~W

1:

30.

'=

'O\W

'=

'O%W '=

10.

,,-

"-,

,,-

I

]

1.11 .1.~3 1~79 2.19 2.64 3.12 3.63 4.1,9 4.78 5.41 6.79 7.54 8.32 9.14 10.0 10.9 11.8 13.8

1

1

".

I

,,-t

~ 30.IO%W Ii-

""I

II"

Go

+ 5/8 3/4 7/8 I 11/8 11/4 13/8 1 1~ 15/8 1 3/4 2 21/8 21/4 23/8 21/2 25/8

.854 1.15 1.48 1.86 2.28 2.74 3.24 3.78 4.36 4.99 6.35 7.10 7.88 8.73 9.60 10.5

.501 ~ .805' 1.18 1.63 2.14 2.73 3.39 4.12 4.92 5.80 7.76 8.85 9.99 11.3 12.5 13.9

1.45 1.95 2.50 3.13 '~83 4.59 5..2 6.31 7.28 8.32 10.6 11.6 12.1 14.5 15.9 11.5

1.39 1.79 2.22 2.70 3.22 3.76 4.26 4.99 5.56 6.36 7.90 8.73 9.58 10.5 11.4 12.4 ,"-

2 3/4 3

11.5 13.5

15.3 18.4

19.0 22.4

13.4 15.6

1.52 1.89 2.29 2.72 3.11 3.55 4.15 4.67 5.22 5.80 7.02 7.67" 8.33 9.04 9.66 '10.5-e

1.09 1.45 1.99 2.30 2.79 3.31 3.88 4.4' 5.14 5.83 7.33 8.05 9.00 9.91 10.9 11.8

11.3 12.9

12.8 15.0

1.15 1.49 1.85 2.23 2.63 3.06 3.52 3.99 4.49 5.02 6.14 6.74 7.35 8.00 8.66 9.35 10.1 11.6

300 reinforcement

.427 .616 .901 1.~ lcc 1.39 1.71 2.07 2.46 2.89 3.35 4.38 4.94 5.54 6.18 6.85 7.55 8.28 9.85

7.5-10

/

Joint Design and Production

Notice that the decreased arc time with the E-6024 results in a slightly lower operating factor, 43.5% instead of 50%, although the joint does cost less.

study of the job, which we are trying to avoid. The nomograph, Figure 6, may be used to quickly 'read the labor and overhead cost per foot of weld.

One might further suggest using a downtime per electrode and a handling time per foot of weld. These figures, if available, would give a more true picture of the welding cost, but it would mean making a time

4. COST PER HOUR As a matter of interest, consider the cost per hour for these two procedures:

E-6012 ELECTRODE rod consumedper hr

E-60a4 ELECTRODE !it!' rod con~umedper hr ..

GOOOM(OF) -(6000)(73/4)(50%)

GOOOM(OF)- (6000)(10.2)(43.5%)

NLmEa -(219)(16)(90%) = 7.37 lbs/hr rod cost

rod cost

7.37 x 14. 9 ~/lb = $1.10/hr labor cost

NLmEa -(218(16)(90%) = 8. 49 lbs/hr

= -!! Q.Q. Total = $7. 10/hr

It can be expected then that the cost per hour for making the same size weld will increase slightly with faster procedures. Obviously the increase equals the difference in cost of electrode consumed. Of course the number of units turned out per hour is greater, so the unit cost is less. ;, 5. ESTIMATING ACTUAL WELDING TIME ~ After the length and size of the various welds have been determined, there are three ways to estimate the actual welding time: 1. Convert these values into weight of weld metal per linear foot, and total for the entire job. Determine the deposition rate from the given welding current, and from this find the arc time. This method is especially useful when there is no standard welding data for the particular joint. 2. If standard welding data is available in tables, giving the arc travel speeds for various types and sizes of welds, in terms of inches per minute, apply this to

8.49 x 16. 9 ~/lb = $1. 44/hr labor cost

= -!! Q.Q. Total = $7. 44/hr

the total lengths of each type and size of weld on the job. 3. Time the actual weld or job. Most welding procedures are based on good welding conditions. These assume a weldable steel, clean smooth edge preparation, proper fit-up, proper position of plates for welding, sufficient accessibility so the welding operator can easily observe the weld and place the electrode in the proper position, and welds sufficiently long so the length of crater is not a factor in determining weld strength. Under these standard conditions, the weld should have acceptable appearance. Failure to provide these conditions requires a substantial reduction in welding current and immediately increases cost. It is impossible to put a qualitative value on these factors, therefore the designer or engineer must learn to anticipate such problems and, by observation or consuIting with shop personnel or other engineerswho have actual welding experience, modify his estimate accordingly.

7,5-12

/

Joint Design and Production

P DB If JBA

Q

0

I

-I()

-,

0

PBI!W!IUn

~

"J

..

SUD!I!SDd

,~

IIV I

~

~

puDIDI:!

"J ~

W'\

..,

~

~

':: 1 0.&.

-y

-I()

0

C'I

0

C'I

0

"-

(')

..,.

I()

"-

~

0

C'I

(')

I() (')

(') 0-

0 ~

"~

0 C'I

0"-

I() "-

0-0

"-

0 (')

"-

~'0

on

on ~

on

~

~.,

,

w~

'6

'

~

-U

~.- ~ t-

..~...

N

S -~

~

.-'

"

",

"

~N

-'

o~

",

~

'"'

,

'

~

--'"'

C'lC'lC'lC'I(')

'-v.

~

~

,

,

.t

on

~

.-

I

0

. "!

=3; « .

* «

7.6-2 /~ Joint Desi and Prod Appr dista of 650 isot from wal pa or for str it is d to r t z.o me of de lo st o t p o l'.§ ma to be ad If ne th n J.8 sh be p fo a u str eq the al un s in o 1-7 mi th de lo I~ me /.5 1.4 I Pro 1 I l.3 To rei an ex m to w a ~ /.2 tion loa of 20 lb T e s h a /./ cro ar 10 in a w ing str of 0" = 18 ps Th o d lo 1.0 de (D liv (L an im ( t f I ..9 low ~ .8 DL for 10 Ib -': 1 i = 1 p n .7 LL + I 80 8 .6 Ll + for 18 ~ond p ~ ~ ~ .4 Th me m n be in i s f n o .3 an ad 20 lb of liv lo (L .2 All str in or m = 1 p De loa To be us ne st to be ad 8 Y4 rz" 03~" I" I~" Irz" /01 Z" 20 Ibs plata thick (t) inch ' = 2. = ar of n s t b a o 8,0 FIG. 3 A guide to esta prop wel proce for mini hea inpu Ch thi as fo DL for 10 Ib -': 1 i = 1 p n oppo direc as the appl load to the bea LL ~ ! fO 1 8 0 If the weld were done alon the to! flan onl + I 20 this would tend to disto bea dow in the an p ~ ~ same The it mig be well, in some case to tem ~sh up a beam order redu som or all of the bea loa while weld f~" 3. AWS AISC AND AAS SPE ~: Secti 7 of the pres A WS Cod for We in Build Cons and the Spe for ~ ~ Weld High and Rail Brid cov the 1 p stren and repa of exis stru 8 P The engin shal dete whe or not a mem is perm to carr live load stre whi weldi or oxyg is bein perf on it, tber's ng mto conS erah e ex en 0 w IC e me 12 in @ 8 p ~ . 0 . , cross is heat as a resu of the ope being perfo 2 If mate adde to mem carr a dea load stres of 3000 psi, eithe for repa cor FI 4 I

I&..

0

-3

.5

¥)

./

aki

'

..

d

'

th

t

t t

h. h th

10.0

-.".

r

In.

@

10,000

pSI =

100

.

7.6-4

/

Joint Design and Production

There is little chance that the structure to be repaired is made of wrought iron, which was used in structures prior to 1900. Wrought iron contains slag rolled into it as tiny slag inclusions or laminations, and is low in carbon. The slag pockets might bother the welding operator a little, but this should be no real problem. Some engineers recommend that extra effort be made to fuse or penetrate well into the wrought iron surface, especially if the attached member is going to pull at right angles to the wrought iron member; otherwise, they reason,the surface might pull out becauseof the laminations directly below the surface. It is also possible for the sulphur content of wrought iron to be excessive,and it should be checked. Keep in mind that any chemical analysis for sulphur represents the average value in the drillings of steel taken for analysis. It is possible in wrought iron to have the sulphur segregated into small areas of high concentrations. The low-hydrogen electrodes (EXX15, EXX16 and EXX18) should be used where sulphur might be a problem. The AISC published in 1953 a complete listing of steel and wrought iron beams and columns that were rolled between 1873 and 1952 in the United States. S. TEMPERATURE FOR WELDING The AWS Building and Bridge codes require that welding shall not be done when the ambient temperature is lower than 0°F. When the base metal temperature is below 32°F, preheat the base metal to at least 70°F, and maintain this temperature during welding. Under both codes, no welding is to be done on

metal which is wet, exposed to ice, snow, or rain, nor when the weldors are exposed to inclement conditions, including high wind, unless the work and the weldors are properly protected. In general, the AISC and AWS specifications on minimum temperature for welding are a good guide to follow. See Table 1. The following thoughts might supplement them in producing better welds at these cold temperatures. Welding on plates at cold temperatures results in a very fast rate of cooling for the weld metal and adjacent base metals. With thicker sections of mild steel, A7, A373, and A36, this exceptionally fast rate of cooling traps hydrogen in the weld metal. This reduces ductility and impact strength of the weld and may cause cracking, especially of the root bead or first pass. This type of weld cracking has been shown to occur almost entirely in the temperature range below 400°F. With a preheat or interpass temperature of 200°F, this cracking does not occur, even with the organic type of mild steel electrodes. This is because the higher temperature results in a slower cooling rate, and more time for this entrapped hydrogen to escape. Low-hydrogen electrodes greatly reduce the source of hydrogen and, therefore, the cracking problem. This weld metal has greater impact strength and a lower transition temperature. In general, the use of lowhydrogen electrodes will lower any preheat requirement by approximately 300°F. The fastest cooling rate occurs with so-called "arc strikes", when at the start of a weld the electrode is scratched along the surface of the plate without any metal being deposited. This can be damaging and

"

.

TABLE l-Minimum

Thickness of Thickest Part at Point of Welding, in inches

Preheat and Interpass Temperatures 1. 2 Welding Process

Shielded Metal-Arc Welding with Other than Low-Hydrogen Electrodes ASTM A363, A73.4,

To 0/4, incl. Over 34 to 11/~, incl. Over 1~/.2 to 2/2, incl. Over 21/2

A3733

Shielded Metal-Arc Welding with Low-Hydrogen Electrodes or Submerged Arc Welding ASTM A36",

A74.",

None7 150°F 225°F 300° F

1 Welding shall not be done when the ambient temperature 2 When the bose metal is below the temperoture listed

A373",

A441"

None7 70°F 150°F 225° F is lower than Oaf. for the welding process

being

used

ond

the

thickness of material being welded, it shall be preheated for 011 welding (including tack welding) in such manner that the surfoces of the parts on which weld metal is being deposited ore at or above the specified minimum temperature for a distance equal to the thickness of the part being welded, but not less thon 3 in., both loterolly ond in odvance of the welding. Preheat temperature shall not exceed 400°F. (Interpass temperature is not subject to a maximum limit.) 3 Using E60XX ar E70XX electrodes other than the low-hydrogen types. 4 See limitations an use af ASTM A7 steel in Par. 105(b). " Using law-hydrcgen electrodes (E7015, E7016, E701B, E702B) or Grade SAW-lor SAW-2. " Using only low-hydrogen electrades (E7015, E7016, E701B, E702B) or Grade SAW-2. 7 When the base metal temperature is belaw 32°F, preheat the base metal to at least 70°F.

1.1-4

/

Joint Design and Production

For (c),

5. TRANSVERSE SHRINKAGE

~=

Transverse shrinkage becomes an important factor where the net effect of individual weld shrinkage can be cumulative. The charts in Figure 8 throw some light on transverse shrinkage. In the lower chart transverse shrinkage, for a given plate thickness, is seento vary directly with the cross-sectional area of the weld. The large included angles only help to illustrate this relationship and do not represent common practice. The relative effects of single and double V-joints are seen in the upper chart. Both charts assume no unusual restraint of the plates against transverse movement. Calculations show that transverse shrinkage is about 10% of the average width of the cross-sectionof the weld area.

V

(35 v) (~10 amp) ( 60) 8 ' /min = 81,000 Joules/linear in. of weld

Another condition can be observed by using conditions (a) and (b) of Figure 7. Two butt joints were made, one in the vertical position and the other in the horizontal position, using a ~ultiple-pass groove weld. The same welding current (170 amps) was used in both joints. The vertical joint used a vertical-up weaving procedure, 3 passes at a speed of 3" /min., procedure (a). The horizontal joint used a series of 6 stringer passes at a speed of 6" /min., procedure (b). The faster welding of (b), 6" /min., produces a narrower isotherm. However, it required 6 passes rather than 3 of procedure (a), and the net result is an over-all cumulative shrinkage effect greater than that

Atrans= .10 ~

t

for (a). This helps to explain why a given weld made with more passeswill have slightly greater transverse shrinkage than one made with fewer passes.The transverse shrinkage can be reduced by using fewer passes. A

= .10 X aver. width of weld Where the submerged-arc process is involved, the cross-sectionof the fused part of the joint is considered rather than simply the area of the weld metal deposited.

further reduction can also be achieved by using larger electrodes. In the weld on sheet metal, Figure 7 (d), it is

II' Problem .u...~... 1.I I

noticed a greater portionto of adjacent base. metal is that affected as compared thethe weld itself. This combined with the fact that the thin sheet metal is les~ rigid than the thick plate (its rigidity varies as its

. Estimate the transverse shrmkage to be expected after welding two 1" plates together if plates are free to pull in.' Use a double-V groove weld, Figure 9.

thickness cubed), helps to explain why sheet metal always presents more of a distortion problem.

600

~~ ~.15 ~ "t

g.ID § ~-

R.

~.-

!

vf. 4t. ~

4t" ~. 1$ r

Plat.th,CX"...5 ("",h.5)

Tran..v~rs~ contraction-.5Iilgl. Vro:Dwble V

--!

~ Ye"

./0

~ ~J5

FIG. 9 Transverseshrinkage of this weld can be closely estimated from computed crosssectionalarea of the weld.

'"~

]JO ~ :!~

~.

area of weld

~ "'./0

.lO

..60

.40

c,.oo" "tiona.1ar.a. of weld("'luo.re"",he.)

Transversa contraction -constant platathlcknass A:{ FIG. 8 Transverseshrinkage varies directly with amountof weld deposit.

(¥S")(1")

= .125

2( Ih) ( Ih") ( .58") = .29 2(%)(1")(%6")

= ~ Aw = .498 in.2

7,7-8

/

Joint

Design and

Production

~ ~

(a)

temp~rature

during w~/di"g, top

expands

-center

bows tJp

o'/6trib/ltion. Cross

.uction

j.

L

=__J'

@)@

shortly a.fter w~/dinfJ-~:)$;,;~ still bowed up .slightly

(b)

,-'

Q) @

FIG. 16 Proper welding position and sequence for fabrication when girder is supported by inclined fixture (top) or trunnion-type fixture

I

-,Y.-- l-

-~ after

/c ) I'

(bottom).

T

coo/ad -e'nci.s "e'ry

.S"lightly boll.C'd up cillt' to contraction of top

I"M -al,mum

r::::=w

FIG,

15 ..aM

To

avoid

bowing

of

long,

thin

box

if:.

.I

W

f:.

'

sections welded up from two channels, the first weld is protected against cooling until the second weld is completed, The two welds are

rr

-I

C_oed Warpaqe T," of Flanqe

latealD I !Bet Co t I , el,a,onwe'" n.,.. rIWeb CJId ~ atFI- at

~

~'f

2

Tillaf Flanqe

Canta't Surface

w.ri"'ge alFlanqt

then allowed to cool simultaneously. weld on the since opposite side, weld usuallymay results in some final bowing the second not quite pull

IiI

the member back, Figure 15. Notice (a1) the heatin g of the top side of the member by the first weld initially causes some expansion and bowing upward. Turning the member over quickly while it is still ~n this shape and depositing the second weld, increases the shrinkff f mg e ect 0 the second weld deposit and the member .., IS usually straight after coolmg to room temperature.

l~tI) I I '

.

The f

sequence

for

automatic

welding

to

produce

the our fillets on a fabricated plate girder can be varied without major effect on distrotion, In most cases this Sequenc e is based 0 the type of fixt n ure used and the method of moving the girder from one I h (FIg. ' 16) Wh ld ' ,. we mg posItIon to anot er .en a smg e automatic welder is used, the girder is usually positioned at an angle between 300 and 450 permitting th ld be d fl ' ...Devi!Jfi"n e we s to eposrted m the at posItIon. ThIS

.

..

position .dopfhs

is

desirable

since

it

makes

slIghtly faster. It also permits

welding

easier

and

better control of bead

shape and the production of larger welds when necessary. Permissible A WS tolerances for most welded

~~§~~~~~~§~~ L(letll

I

L

De,ialion F,am Specifi.d Combe, atWelded Girde" f:.~' t~tMltNa' l..sl~n'~'

: I

~=:===== ~;;~;~~~~ =====~:J

De,iafian Fram Sfraiqhlne.. It Wtldod Calymn. L"'qlhs 0145ondUnder 6(;nch.s) .~tMlt Not O,er f

=

L (fetf)

1

LenathsO,er .45' f:.linchesl'~

Swoopof Welded GirdtrS A inch.. ' to

Intermediate Stiffeners onBoth S,dts ofWeb: If t ' Less Thon & f:..-~ "&ormort A'-& In1ermedialeSliffe..rsonOneS~eoIWeb: "~ssTh~-&c,.-& t, ;&orMore c,.- & ItIlntermed;ole Stiff",.rs c,=- & f,omSpecified Dopth of weldod Gi'der Measured otwebCellterli.e depthS up to 36' ine! aye, 36"to 72' i~1

dtpthsOIle,72"

t i .O.,iot;on t ;\' Bttwte.

+ ~"-'"

FrO"'Flotness01 GirderWebin a lo"lth Stiffe.oro Dr"a LRnqlh EQuolto

Depthof Girder

FIG. 17 AWS permissible tolerances for common welded members.l